Variable frequency indexer



April 29 1958 Filed Sept. 30. 1955 H. M. GRAIN QELHFI QEE EIUQEIII VARIABLE FREQUENCY INDEXER 2 Sheets-Sheet 1 FIG.I

REFERENCE FREQUENCY INPUT RIO VIO9

VARIABLE FREQUENCY INPUT INVENTOR HARRY M. GRAIN ATTORNEYS April 29, 1958 H. M. CRAIN VARIABLE FREQUENCY INDEXER 2 Sheets-Sheet 2 Filed Sept. 30, 1955 R N o .1 mm W M W I c M m w M J B J WW1 x 3m 30 S O v 9.

ATTORNEYS United States Patent VARIABLE FREQUENCY INDEXER Harry M. Crain, Philipsburg, Pa., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application September 30, 1955, Serial No. 537,910

2 Claims. (Cl. 324-57) The present invention relates to a variable frequency indexer and more particularly to a marker signal generator for automatically indexing a frequency response curve at specific predetermined intervals.

Prior art marker signal generators indicated a mark on both sides of the point at which it was desired for the mark to appear. These marks were not adjustable to allow for inherent delays where a mechanical system was used to record the frequency response plot. The above marks also were restricted to a particular purpose and were not amendable to adjustment of the marker to any desirable frequency interval.

Accordingly, an object of the present invention is to provide markers at specified intervals of frequency on a plot of the frequency response of an electrical circuit and to eliminate the necessity of and errors inherent in a recording system that must be synchronized with the applied varying frequency.

Another object is to provide an apparatus capable of indexing a mark that may be varied to any desirable interval of frequency on a frequency plot.

A further object of the invention is the provision of a system having a marker signal that is adjustable so that compensation may be made for any inherent delay in the recording of the marker signal.

Still another object is to provide an apparatus having a high degree of accuracy and adjustability in the production of markers on a frequency response plot.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Figs. 1 and 2 are to be taken together in side-by-side relationship and show a schematic diagram of a preferred embodiment of the apparatus of the invention.

Referring now to the drawings, wherein like reference characters designate like or corresponding parts, there is shown schematically in Figs. 1 and 2 the electrical circuit of the present frequency indexer, wherein the power supply may be a conventional one operated from a suitable current source and which provides the necessary operating voltages to operate the vacuum tubes described elsewhere herein. In this embodiment of the invention, there is used a power transformer T1 to provide the necessary voltage to operate a full wave rectifier tube V110 and to supply the heater current for all the vacuum tubes from terminals 1 and 2. Tube V110 is connected in the conventional manner to the output of the power transformer T1, the output of tube V110 is taken at the cathode and fed into a conventional filter circuit. The filter circuit contains four resistors R100 through R103 in series tapped off the cathode of tube V110; five capacitors C100 through C104 to ground which are connected to each resistor to provide 21 pi filter network; and a gas tube V111 is connected in parallel across the last capacitor C104. The plate voltage for V107 is taken Off between the first and second filter resistors R100 and R101, the plate voltage for tubes V104, V105, and V106 is taken off between the third and fourth filter resistors R102 and R103, and the plate voltage for tubes V101, V102, V103, V108 and V109 is taken off from tube V111 to insure a constant voltage which is deemed necessary for the proper operation of these tubes.

The variable frequency input is taken from socket S2 and is applied across the cathode resistor R81 of tube V108(A) and ground with an isolation capacitor C81 in the positive line to prevent direct current from being introduced into the circuit. The grid of tube V108(A) is tied directly to ground, line G, and bias is obtained by the use of a cathode resistor R81 of appropriate value. The plate circuit contains a plate load resistor R and the plate is capacitively coupled to the grid of tube V108(B) by capacitor C82. The plate of tube V108(B) is connected directly by line H to the positive voltage or the B+ of the power supply as is conventional with a cathode follower stage. The cathode of tube V108(B) is connected to the cathode of tube V109(A) and to ground through a resistor R91 of low resistance. Tube V108(B), then, is a cathode follower stage and it provides a low impedance drive to tube V109(A) which is necessary to obtain a flat frequency response. Tube V109 is a dual triode and is connected in the same manner as tube V108 with the output being taken from the cathode of tube V109(B) which is connected to the cathode of tube V103(A) and to ground through a cathode resistor R33 in the cathode circuit of tube V103(A).

The operation of this circuit is as follows: V108(A) is an amplifier and its output is connected to the grid of V108(B) which is a cathode follower whose output is similar to its input. The main purpose of V108(B) is to provide a low input impedance to V109(A) because said V109(A) has input capacitance and a low input impedance to this tube is necessary to obtain a fiat frequency response in the higher frequency ranges. V109(A) is an amplifier and further amplifies and/or clips the input signal depending on its amplitude. V109(B) is a cathode follower similar to V108(B) and provides a low input impedance to V103(A) for the same reasons given for V108(B). When a signal of small amplitude is applied to the cathode of V108(A), it will be amplified. If the amplified signal is supplied in the same form to the cathode of V109(A), it will be further amplified and clipped. The output of V109(B) will then be applied to V103 (A) as a clipped sine wave of constant amplitude. If a large signal is applied to the cathode of V108 (A), it will be amplified and clipped and then applied to V103(A) in substantially the same form and amplitude as the previously described small signal. Therefore, any variable input signal of .3 volt R. M. S. ,to 10 R. M. S. will have the same form and amplitude when applied to the cathode of V103(A).

The same result for various Values of variable frequency input signal is due to the fact that the variable frequency input signal applied to V109(A) and V109(B) is always of such value as to drive V109( B) past saturation; hence the amplitude of the output of V109(B) is the same for all values of input signal within the limits described.

The reference or indexing frequency of one kilocycle, which may have a magnitude of substantially three volts R. M. S., is taken from socket S1 and is fed to the grid of tube V101. A grid current limiting resistor R12 is connected between the frequency source and the grid and a grid return resistor R10 is provided between the grid current limiting resistor R12 and ground. Tube V101 is a conventional resistance coupled amplifier, a portion of whose plate resistor is common with the plate .of V102(A), that part not being common acting as an V102 by tube V101. The grid of X102(A) is tied directly to ground, the cathodes of tube V102 (A and B) are tied together and to ground through a cathode resistor R23 and by line H to the posiitve voltage B+ of the power supply through another resistor R31 and to the grid of tube V103(A). The plate of V102(A) is coupled to the grid of V102(B) through a coupling capacitor C22 and from there through a resistor R22 of high resistance to B+ of the power supply by line H.

The plate circuit of V102(A) contains a plate load resistor R20 while the plate of V102(B) contains a plate load resistor R21 and a capacitor C21 in parallel of such value that tube V102 will tend to operate as a relaxation oscillator at a frequency near that of the reference frequency. Tube V102 would be a conventional free running multivibrator except for the capacitor in the plate circuit of V102(B). It is well known in the art, that a conventional multivibrator gives an output waveform having a small peak or overshoot at the end of each pulse. The capacitor C21 in the plate circuit of V102(B) is for the purpose of emphasizing this overshoot. With the circuitry as described above, each pulse of tube V101 triggers tube V102 which then causes pulses of current to flow through it at the same frequency as the indexing frequency. The plate and grid resistances and capacitors are chosen of such value as to cause the mono-stable multivibrator to operate at a frequency near the indexing frequency so that when the indexing frequency is applied to the input of this tube it causes it to fire at exactly the rate of the indexing frequency and locks it in at that frequency so long as it is applied.

The grid of V103(A) is connected to the cathode of V102(B). The cathode is connected to the cathode of V109(B) through a resistor R33 to ground and to B+ through a resistor R32, to maintain V103(A) cut 01f in the absence of proper signal conditions. The plate is connected to B+ through a resistor R30 and capacitor C30 in parallel and to ground through a capacitor C35 and a rseistor R35 of high resistance having a variable tap connected to the grid of V103(B). The cathode of V103(B) is connected to ground through a resistor R36, the plate of V103 (B) is connected to B+ through a resistor R34 and a capacitor C34 in parallel and by line F to the grid of V104(A) through a coupling capacitor C41. The bias on the cathode of V103(A) is obtained through the current of V109(B) through the cathode resistor R33 in the cathode circuit of V103(A), and the current through the resistor R32 connected from B+ to the cathode and the cathode resistor. The values of these resistors are chosen such that V103(A) is cut off until the spike of the output waveform of V102(B) and the maximum negative portion of the output waveform of V109(A) coincide in time. The bias on the cathode of V103(A) must be such that the maximum negative amplitude of V109(B) on the cathode of V103(A) is just short of that necessary to cause conduction of V103(A) and consequently, when the spike of the waveform from V102(B) is applied to the grid of V103(A), it, in combination with the voltage from V109(B) applied to the cathode of V103(A), is sufiicient to cause V103(A) to conduct for the duration of the spike thus causing spikes of current to flow in the plate circuit. Due to the relatively long RC time constant in the plate circuit, these spikes of current are integrated and the Waveform which appears at the grid of V103(B) is in the general form of a sine wave, but constantly varies slightly in amplitude. The frequency of this waveform at the plate of V103(A) will vary at the rate of Af which is the frequency difference between the variable input frequency and the closest integral harmonic of the indexing frequency. If the frequency of the clipped wave of the variable input frequency signal is doubled, a 1 kc. reference spike will coincide with every other cycle of the clipped wave. If the variable input frequency is .4 tripled, then every third cycle has a reference spike or mark. As long as the amplitude of the reference spike is constant and its duration less than one half cycle of the variable frequency, say 2 microseconds, the voltage amplitude appearing at the plate of V103(A) will be constant for any A over the operating range from 1 kc. to 200 kc. of the variable input frequency. Because of the limiting action of tube V108 and tube V109, the input variable frequency amplitude can vary from .3 volt R. M. S. to 10 volts R. M. S. without affecting the operation of this device.

The cathode of V103(B) is connected to ground through a resistor R36, the plate is connected to B+ of the power supply through a resistor R34 and capacitor C34 in parallel and is also connected to the grid of V104(A) through a coupling capacitor C41. The grid of V103(B) as previously mentioned is connected to the variable tap of the resistor R35 connected to the plate of V103(A) through a capacitor C35 and to ground. Variation of this tap varies the signal level on V103(B). Due to the RC time constant in the plate and the amplification by the tube, the waveform is further amplified and integrated, the output being an amplified and more smooth waveform having the characteristics of a sine wave.

The grid of V104(A) is connected to the coupling capacitor C41 and to ground through a resistor R41. The cathode of V104(A) is connected to ground through a cathode resistor R42. The plate is connected to B-I- through a resistor R40 and capacitor C40 in parallel and to the grid of V104(B). The plate of V104(B) is connected to B+ through a resistor R43 and through a capacitor CS1 to the plate and grid of V(B) which is in turn connected to ground through a resistor R51. The grid of V104(B), as previously mentioned, is connected to the plate of V104(A). The cathode of V104-(B) is connected through a resistor R44 to ground and through a capacitor CS0 to the plate and grid of V105(A) which are connected through a resistor R50 to ground. V104(A) further amplifies the input signal to it and further smooths it due to the RC time constant in the plate circuit. The substantially pure sine wave that appears in the output is then applied to V104(B) which is a phase splitter and takes the place of a transformer, and feeds tube V105 which operates as a full wave rectifier.

The cathodes of tube V105 are tied together and to ground through a resistor R52 and also to the grid of V106(A) through a resistor R61. The output of tube V105 is then a rectified full wave and is applied to the grid of V106 which is a one-shot multivibrator.

The grid of V106(A) is tied to the plate of V106(B) through a capacitor C62, the plate of V106(A) is tied to the grid of 106(B) through a capacitor C63 and t0 B+ through a resistor R60 and the cathodes of tube V106 are tied together and connected to ground through a resistor R64. The plate of V106(B) is connected to B+ through a resistor R62 and also is connected through a capacitor C70 to the grids of tube V107 which are tied in parallel.

In order to explain the operation of the tube V106 it is necessary to retrogress. In practice, some time delay occurrs between the electrical indexing signal and the variable marking or indication of this point. If the device were set to be triggered at exactly 1 kc. intervals, the actual indication of this point would occur at some later point; hence it is necessary to cause the triggering to be done just previous to this point to compensate for this inherent delay.

As the index point is approached electrically the signal output of tube V103 (A) varies from 500 C. P. S. to zero beat. Since V103(B) and V104(A) have capacitive plate loads, a rising amplitude of their output is affected as the frequency decreases. For a particular triggering threshold of tube V106, one can select a frequency of proper amplitude prior to the zero beat point that will trigger V106 and compensate for the delay mentioned previously. However, for any particular frequency chosen, an amplitude of signals suflicient to trigger tube V106 will occur on both sides of zero beat. Since it is desirable that tube V106 be triggered on the first impulse but undesirable that it also be triggered on the second one, this latter pulse must be nullified. This condition is met by providing an RC time constant in the grid circuit of tube V106 that prevents tube V106 from firing again until the undesirable frequency has been passed, but is short enough to allow recovery for the next desired index point. This condition is obtained because of the inherent operation of tube V106 as a single shot multivibrator. Due to the positive bias on the grid of V106(B), this tube is normally conducting. The conduction of V106(B) causes a positive bias to be applied to the cathode of V106(A) such that it is normally cut off. When the signal level on V103(B) is adjusted by the variable tap of resistor R35 to provide a signal of sufficient amplitude when applied to the grid of V106(A) to overcome its bias, it will begin to conduct, it will reach saturation and will remain conducting for a length of time depending on the RC time constant of the grid circuit. This time is adjusted so that V106(A) remains conducting until just after the second impulse of equal magnitude that occurs after zero beat is past, but is short enough to allow tube V106 to revert to its normal operation condition before the next pulse occurs. The conduction of V106(A) causes the pulse potential on its plate to drop which in turn reduces the positive bias on the grid of V106(B) sufiiciently to cause it to stop conducting. When V106(B) stops conducting, the plus potential on the plate risescausing a plus potential to be applied to the grids of tube V107. This causes tube V107 to conduct only on the desired pulse, thus causing an indexing mark or indication to be made.

Tube V107 is a dual triode power amplifier or relay driver which consists of two grids connected together and to the plate of V106(B) through a capacitor C70 and also connected to ground through a resistor R70, two plates connected together .and directly to B+ and also to ground through two resistors R71 and R72, the two cathodes are connected together and to a relay Y shown in the non-operative position in Fig. 2 which is in turn connected between the two resistors previously mentioned in the plate circuit of V107. Conduction of tube V107 actuates the relay in its cathode circuit to close the relay contacts which activates the marking or indicating mechanism connected across the output at socket S3 which causes the frequency indexing mark to be made or to appear on the visual indication of the frequency response of the electrical component or circuit being tested.

Test points for the circuit are located at terminals 3 and 4 to ascertain whether the correct signal level appears at V103(B) so that proper adjustment of resistor R35 may be effected accordingly.

In one embodiment of theinvention shown merely by way of example and to be considered nowise as limiting the scope of the invention, the following values of the various electrical elements were found to be appropriate:

6 R35 2 megohms. R36 9.1 K ohms. R40 220 K ohms. R41 2.2 megohms. R 9.1 K ohms. R43 47 K ohms. R44 47 K ohms. R50 220 K ohms. R51 220 K ohms.

R52 lmegohm. R60 47 K ohms. R61 2.2 megohms. R62 47 K ohms. R63 2.2 megohms. R64 2.4 K ohms. R70 2.2 megohms. R71 100 K ohms. R72 1.5 K ohms. R 47 K ohms. R81 1.2 K ohms. R82 100 K ohms. R 47 K ohms. R91 510 ohms. R92 K ohms. R100 l K ohms, 10 watt. R101 1 K ohms, 10 watt. R102 1 K ohms, 10 Watt. R103 5 K ohms, 10 watt.

T1 Power Transformer Chicago PSC-85. S1 AN-3102-16S-5P. S2 AN-3102-16S-5P. S3 AN-3102-16S-5S. L1 Hubbell Twist Lock. Y Relay Allied BJ 6D 82 v.

Capacitors Mica cap.+10%

C13 mfd 0.5 C21 mmfd 100 (22 mfd .01 C30 mfd .01 34 mfd .01 C35 mfd 0.1 C40 mfd .01 C41 mfd 0.1 C50 mfd 0.1 C51 mfd 0.1 C62 mfd 0.1 C63 mfd .022 C70 mfd 0.5

C81 mfd 0.5 C82 mfd .01 C92 mfd .01 C100 1 mfd 20 C101 1 mfd 20 C102 1 mfd 20 C103 1 mfd 20 C104 mfd 1.0

1 Filter capacitors are dual 20-20 at 450 v. D. C.

Tube No. Tube Type Function Amplifier-Driver. Multivibrator. MixerAmpli.fier. Amplifier-Phase Splitter. Signal Rectifier. Multivibrator. Relay Driver. Limiter.

Do. Power Rectifier. Regulator.

Accordingly, it is therefore apparent in the light of the foregoing description that the above invention presents a variable frequency indexer or generator providing a frequency mark which may be varied to any desirable interval and may be adjustable to compensate for any inherent delay in causing the mark to be made.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A variable frequency indexer for indexing a frequency response curve of an electrical circuit being tested by providing indicating marks at a predetermined adjustable interval of frequency on the plot of the frequency response curve comprising a first amplifier, a first multivibrator connected to said first amplifier, a reference frequency voltage being applied to said first amplifier, the output of said first multivibrator being a spike signal synchronized to said reference frequency voltage, a first limiter, a second limiter connected to said first limiter, a variable frequency voltage being applied to said first limiter, the output of said second limiter being a clipped sine wave signal of the frequency of said variable frequency voltage, a mixer, said spike signal and said clipped sine wave signal being fed to said mixer so that a third signal is produced when a spike signal and a clipped sine wave signal coincide, said third signal occurring at specific predetermined intervals dependent upon the frequency relationship between the reference frequency voltage and the variable frequency voltage and said intervals being adjustable by changing the frequency of the variable frequency voltage, said third signal beinga signal of a sine I wave characteristic having a frequency varying at the rate depending upon the difference between. the frequency of the variable frequency voltage and the closest integral harmonic of the reference frequency voltage, means. for selecting a signal level of a particular amplitude and duration of time ahead of the zero beat of said third signal, second and third amplifiers connected to said mixer and amplifying and smoothing said third signal, a phase splitter connected to said third amplifier, a second full wave rectifier electrically following said phase splitter, a second multivibrator connected to said rectifier, a relay driver coupled to said second multivibrator, a relay in the output circuit of said relay driver, said amplified third signal of said third amplifier being applied to said phase splitter,

the output of said phase splitter being fed to said signal rectifier, the output of said signal full wave rectifier being fed to said second multivibrator to fire said second multivibrator so that any inherent delay in the actuation of a marker signal in a marking mechanism is compensated for, the output of said second multivibrator being fed to said relay driver thereby actuating said relay whereby a marking mechanism coupled to the relay is activated causing an indexing mark to be made at the predetermined interval on the plot of the frequency response curve of the electrical circuit being tested.

2. A system for indexing a frequency response curve of an electrical circuit being tested by providing indicating marks at a predetermined adjustable interval of frequency on the plot of the frequency response curve comprising a first amplifier, a reference frequency voltage being coupled to said first amplifier, a first multivibrator connected to said first amplifier, said first multivibrator having a capacitor in parallel with its load resistor so that the output of said first multivibrator is a steep spike signal synchronized to said reference frequency voltage, a first limiter, variable frequency voltage being coupled to said first limiter, a second limiter connected to said first limiter, the second limiter having a bias so that any variable frequency voltage drives it past saturation, the output of said second limiter being a clipped sine wave signal of the frequency of said variable frequency voltage, a mixer, said mixer having a bias so that the mixer is cut off until a spike signal and a clipped sine wave signal coincide in time, said spike signal and said clipped sine wave signal being fed to said mixer producing a third signal, said third signal occurring at specific predetermined intervals dependent upon the frequency relationship between the reference frequency voltage and the variable frequency voltage and said intervals being adjustable to different values by changing the frequency of the variable frequency voltage, said third signal being integrated by a long RC time constant in the. plate circuit of the mixer and having a sine wave characteristic with a frequency varying at a rate depending upon the difference between the frequency of the variable frequency voltage and the closest integral harmonic of the reference frequency voltage, means for selecting a signal level by. choosing a frequency of proper amplitude ahead of the zero beat of said third signal, second and third amplifiers connected to said mixer and amplifying and smoothing said third signal, a phase splitter connected to said third amplifier, a signal full wave rectifier electrically following said phase splitter, a second multivibrator connected to said rectifier, said second multivibrator being a single shot multivibrator which is normally cut oif until a signal of sufiicient amplitude determined by the selecting means is applied to overcome the bias thereon, said second multivibrator conducting until the amplitude of the signal determined by the selecting means decreases past the proper amplitude of the signal level after the zero beat of said third signal, relay driver coupled to said second multivibrator, a relay connected in the output circuit of said driver, said amplified third signal of said third amplifier applied to said phase splitter, the output of said phase splitter being fed to said signal rectifier, the output of said signal rectifier being fed to said second multivibrator to trigger said multivibrator so that any inherent delay in the actuation of a marker signal in a marking mechanism is compensated for, the output of said second multivibrator being fed to said relay driver thereby actuating said relay whereby a marking mechanism coupled to the relay is activated causing an indexing mark to be made at the predetermined interval on the plot of the frequency response curve of the electrical circuit being tested.

References Cited in the file of this patent UNITED STATES PATENTS U. S. DEPARTMENT OF COMMERCE PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,832,931 Harry M, Grain April 29, 1958 It is hereby certified that error appears .in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column *7, line 39,. for "second full" read es signal full s; column 8,.

line 5,. after "limiter, insert em a Signed and sealed this 24th day of June 1958o Attest:

KARL H a 'AXLINE ROBERT C. WATSON Comnissioner of Patents Attesting Officer 

